DIAGNOSIS The reset mechanism on that SmartLight indicator is simple but finicky. The most common cause is the battery compartment. Either the battery is dead, or more often, the tiny plastic tab that acts as the battery contact has gotten bent or isn't making a connection.
FIX You'll need a small Phillips-head screwdriver. Flip the filter lid over and unscrew the battery cover on the SmartLight module. Carefully remove the old coin battery (usually a CR2032). Here's the key step: look at the little metal contact *inside* the compartment, the one the battery touches. Gently bend it upward just a tiny bit with your fingernail or a toothpick to ensure it will spring firmly against the new battery. Insert a fresh battery, positive side up. Before screwing the cover back on, press and hold the reset button for 3-5 seconds. You should see the light cycle or turn green.
IF THAT DIDN'T WORK With the battery out, also check the small plastic button/plunger on the underside of the module. Press it a few times to make sure it's not stuck. Reassemble with the new battery. If it still won't reset, the electronic module itself may have failed from moisture exposure. The lid is still perfectly usable; you'll just have to track filter changes manually on your calendar.
HEADS UP When you screw the battery cover back on, don't overtighten it. Snug is fine. Cranking it down can crack the plastic or warp the seal, letting moisture in which will kill the new battery quickly.